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Layered transition metal oxides are the most important cathode materials for Li/Na/K ion batteries. Suppressing undesirable phase transformations during charge-discharge processes is a critical and fundamental challenge towards the rational design of high-performance layered oxide cathodes. Here we report a shale-like NaxMnO2 (S-NMO) electrode that is derived from a simple but effective water-mediated strategy. This strategy expands the Na+ layer spacings of P2-type Na0.67MnO2 and transforms the particles into accordion-like morphology. Therefore, the S-NMO electrode exhibits improved Na+ mobility and near-zero-strain property during charge-discharge processes, which leads to outstanding rate capability (100 mAh g−1 at the operation time of 6 min) and cycling stability (>3000 cycles). In addition, the water-mediated strategy is feasible to other layered sodium oxides and the obtained S-NMO electrode has an excellent tolerance to humidity. This work demonstrates that engineering the spacings of alkali-metal layer is an effective strategy to stabilize the structure of layered transition metal oxides. Suppressing phase transitions is crucial for the layered lithium/sodium transition metal oxide cathodes in batteries. Here, the authors report a water-mediated strategy to mitigate the phase transitions and boost electrochemical performances of manganese-based layered cathodes for cost-effective Na-ion batteries. [ABSTRACT FROM AUTHOR]

Na3VCr(PO4)3 has attracted great attention due to its high energy density and stable structure. The substitution of vanadium with chromium led to an increase in the redox potential from 3.4 to 4.2 V (vs. Na+/Na). In this work, the electrochemical performance of Na3VCr(PO4)3 (NVCP) in Mg cells is successfully investigated. Reversible multielectron Mg2+/Na+ extraction/insertion is observed. Upon charging, NVCP exhibits two plateaus at 1.75 V and 2.3 V vs. Mg2+/Mg0 achieving a maximum reversible capacity of 85 mA h g−1 (a 1.45 electron reaction) enabling oxidation to VV as determined by 51V NMR and XPS, and accompanied by extraction of sodium from Na2 sites according to 23Na NMR. During the subsequent cycles Mg2+/Na+ ions react at 2.3 and 1.7 V, and an additional peak of Mg2+ at 1.2 V is observed. Alternatively, electrochemical cells were also assembled with activated carbon in a 0.1 M Mg(TFSI)2 electrolyte, leading to similar profiles and capacity even at higher current density. Finally, the chemical desodiation of NVCP using I2, Cl2 and NO2BF4 is performed to avoid mobile sodium ions. XRD, XPS and 51V NMR confirmed the contraction of the cell and the oxidation to VIV and VV. Iodine was not capable of full sodium extraction. Despite this fact, the ability of this sample to retain 65 mA h g−1 during a few cycles evidences the reversibility of magnesium insertion. The use of stronger oxidants such as Cl2 or NO2BF4 allowed the increase of the initial OCV and resulted in a plateau at 4 V. The chemical desodiation was also accompanied by an enhanced capacity decrease which could be correlated with the structural degradation. [ABSTRACT FROM AUTHOR]

The recent introduction of glyme-based solvents has opened new opportunities to characterize graphitic materials as anodes for sodium-ion batteries. We evaluated the electrochemical behaviour of a graphitized carbon nanofiber for the first time. X-ray diffraction, electron paramagnetic resonance and nuclear magnetic resonance allowed the sodium insertion mechanism to be untangled, in which the occurrence of an activation process during the first discharge enhances sodium accessibility to active redox centres at the interlayer space. Morphological changes observed by electron microscopy could be responsible for this behaviour. A fully graphitized carbon nanofibers/NaPF6(diglyme)/Na3V2(PO4)3 sodium-ion battery was tested to probe the reliability of this graphitic nanostructure as a negative electrode. [ABSTRACT FROM AUTHOR]

This study reports on the electrochemical alloying-dealloying properties of Mg2Sn intermetallic compounds. 119Sn Mössbauer spectra of β-Sn powder, thermally alloyed cubic-Mg2Sn, and an intermediate MgSn nominal composition are used as references. The discharge of a Mg/micro-Sn half-cell led to significant changes in the spectra line shape, which is explained by a multiphase mechanism involving the coexistence of c-Mg2Sn, distorted Mg2-δSn, and Mg-doped β-Sn. Capacities and capacity retention were improved by using nanoparticulate tin electrodes. This material reduces significantly the diffusion lengths for magnesium and contains surface SnO and SnO2, which are partially electroactive. The half-cell potentials were suitable to be combined versus the MgMn2O4 cathodes. Energy density and cycling properties of the resulting full Mg-ion cells are also scrutinized. [ABSTRACT FROM AUTHOR]

Abstract: A new Na3+xV2(PO4)3−x(SiO4)x (0≤x≤0.5) series is examined as the positive electrodes for sodium‐ion cells. The substitution of P by Si is evidenced by an increase in the lattice parameters and a significant modification of the particle morphology. These structural and morphological changes significantly affect the electrochemical performance. Thus, samples with low Si contents reveal better rate capabilities and cyclabilities than the Si‐free sample. A full ion cell assembled with a selected sample provides an energy density of 342 W h kg−1 after 140 cycles, demonstrating that Si doping on Na3V2(PO4)3 could be an interesting strategy for high‐performance Na‐ion cells. [ABSTRACT FROM AUTHOR]

Off-stoichiometric Na3+3 xV2− x(PO4)3 samples have been prepared by a sol-gel route. X-ray diffraction and XPS revealed the flexibility of the NASICON framework to accommodate these deviations of the stoichiometry; at least for low x values. X-ray photoelectron spectra evidenced the presence of Na4P2O7 impurities. The synergic combination of the structural deviations and the presence of Na4P2O7 impurities induce a significant improvement of the electrochemical performance and cycling stability at high rates, as compared to the stoichiometric Na3V2(PO4)3 sample. The fast kinetic response provided by the induced off-stoichiometry involves a decrease of the cell resistance. [ABSTRACT FROM AUTHOR]

Sodium-deficient nickel-manganese oxides exhibit a layered structure, which is flexible enough to acquire different layer stacking. The effect of layer stacking on the intercalation properties of P3-Na xNi0.5Mn0.5O2 ( x=0.50, 0.67) and P2-Na2/3Ni1/3Mn2/3O2, for use as cathodes in sodium- and lithium-ion batteries, is examined. For P3-Na0.67Ni0.5Mn0.5O2, a large trigonal superstructure with 2√3 a×2√3 a×2 c is observed, whereas for P2-Na2/3Ni1/3Mn2/3O2 there is a superstructure with reduced lattice parameters. In sodium cells, P3 and P2 phases intercalate sodium reversibly at a well-expressed voltage plateau. Preservation of the P3-type structure during sodium intercalation determines improving cycling stability of the P3 phase within an extended potential range, in comparison with that for the P2 phase, for which a P2- O2 phase transformation has been found. Between 2.0 and 4.0 V, P3 and P2 phases display an excellent rate capability. In lithium cells, the P3 phase intercalates lithium, accompanied by a P3- O3 structural transformation. The in situ generated O3 phase, containing lithium and sodium simultaneously, determines the specific voltage profile of P3-Na xNi0.5Mn0.5O2. The P2 phase does not display any reversible lithium intercalation. The P3 phase demonstrates a higher capacity at lower rates in lithium cells, whereas in sodium cells P3-Na xNi0.5Mn0.5O2 operates better at higher rates. These findings reveal the unique ability of sodium-deficient nickel-manganese oxides with a P3-type structure for application as low-cost electrode materials in both sodium- and lithium-ion batteries. [ABSTRACT FROM AUTHOR]

A novel design of a sodium-ion cell is proposed based on the use of nanocrystalline thin films composed of transition metal oxides. X-ray diffraction, Raman spectroscopy and electron microscopy were helpful techniques to unveil the microstructural properties of the pristine nanostructured electrodes. Thus, Raman spectroscopy revealed the presence of amorphous NiO, α-Fe2O3 (hematite) and γ-Fe2O3 (maghemite). Also, this technique allowed the calculation of an average particle size of 23.4 Å in the amorphous carbon phase in situ generated on the positive electrode. The full sodium-ion cell performed with a reversible capacity of 100 mA h g−1 at C/2 with an output voltage of about 1.8 V, corresponding to a specific energy density of about 180 W h kg−1. These promising electrochemical performances allow these transition metal thin films obtained by electrochemical deposition to be envisaged as serious competitors for future negative electrodes in sodium-ion batteries. [ABSTRACT FROM AUTHOR]

The effect of low-level chromium substitution in Na3V2- xCr x(PO4)3 (0≤ x≤0.4), a potential cathode material in Na-ion cells, has been examined. A suitable synthesis procedure is developed to obtain composites of crystalline NASICON phosphate and an amorphous carbon phase to enhance the electrical conductivity of the electrode. The optimized cathode materials were characterized by using X-ray diffraction and spectroscopic techniques. The electrochemical evaluation was carried out by using galvanostatic and potentiostatic methods. The activity of the V5+/V4+ redox couple at approximately 4 V is remarkable for the samples containing chromium. A reversible capacity of 107 mAh g−1 with a coulombic efficiency of 99 % was determined for Na3V1.9Cr0.1(PO4)3 after 40 cycles. The observed performance correlates with a good kinetic response, resulting from low charge-transfer resistance and high diffusion coefficient. [ABSTRACT FROM AUTHOR]

An anatase nanotube array has been prepared with a special morphology: two concentric walls and a very small central cavity. The method used here to achieve the double-wall structure is a single-step anodization process under a voltage ramp. Thanks to this nanostructure, which is equivalent to a fractal electrode, the electrochemical behaviour is improved, and the specific capacity is higher in both lithium and sodium cells due to pseudocapacitance. The double-wall structure of the nanotube enhances the surface of TiO2 being in contact with the electrolyte solution, thus allowing an easy penetration of the alkali ions into the electrode active material. The occurrence of sodium titanate in the electrode material after electrochemical reaction with sodium is studied by using EPR, HRTEM and NMR experiments. [ABSTRACT FROM AUTHOR]

Titania nanotubes (nt-TiO2) with high aspect ratio and amorphous character were prepared by Ti anodization at high voltage (100 V). After annealing, the nanotubes crystallized to anatase phase and the XRD patterns and TEM images showed that self-organized anatase nt-TiO2 are composed of crystallographically oriented nanocrystalline domains on the Ti substrate. The contribution of faradic and pseudocapacitive processes was studied for anatase nt-TiO2 in Na cells obtaining the b-parameter values from cyclic voltammetry experiments. The reaction between anatase nt-TiO2 and Na is dominated by redox surface processes (pseudocapacitance) in the region over ca. 0.5 V and by irreversible faradic processes below ca. 0.3 V. The cycling performance of anatase nt-TiO2 in sodium cell was very strongly affected by the imposed electrochemical conditions. Reversible capacity values of about 150-200 mA h g-1 and good cycling behaviour were found for 2.6-0.5 V of potential limits. Impedance spectra and 23Na MAS NMR spectra were also used to study the reaction mechanisms. In addition, it was found that by using variable voltage during the anodization of titanium, the resulting electrode exhibits excellent cycling behaviour (ca. 190 mA h g-1 after ca. 800 cycles). These results are the best cycling behaviour reported to date for anatase in the form of self-organized nanotubes for sodium ion microbatteries. [ABSTRACT FROM AUTHOR]